The present invention relates to a paper sheet conveyance apparatus and an image forming apparatus provided with the paper sheet conveyance apparatus, and specifically, relates to a paper sheet conveyance apparatus that can convey a paper sheet at a controlled timing and/or in a reversible conveyance mode in which the conveyance direction of the paper sheet is switched from a normal direction to a reverse direction or vise versa and/or in a variable speed conveyance mode, and an image forming apparatus provided with such the paper sheet conveyance apparatus mentioned in the above.
Conventionally, a stepping motor has been widely employed as a driving source of the conveyance mechanism for conveying the paper sheet.
For instance, Patent Document 1 (Tokkai 2005-350245, Japanese Non-Examined Patent Publication) sets forth a stepping motor that is employed as a driving source of the conveyance mechanism installed at the paper sheet re-feeding section of the duplex image forming apparatus, in which an obverse side of paper sheet, having an image already formed on it, is turned to its reverse side, so as to feed the paper sheet again into the image forming section.
It is desirable that the motor for driving the conveyance mechanism is as small as possible, in order to minimize the space occupied by the motor, and further, to reduce its cost.
On the other hand, it is also desirable that a pair of rollers to be configured into the conveyance mechanism has such a sufficient stiffness that an unevenness of the nip portion formed between the rollers does not occur due to the deformation of the rollers. In addition, it is still desirable that the outer diameter of each roller is as great as possible, since it is necessary that the paper sheet should smoothly enter into the nip portion of the pair of rollers.
Generally speaking, since a rated output power of a small-sized stepping motor is relatively small, the step out is liable to occur at its startup time, at a reversing moment of its rotational direction, or at a velocity changing time for its accelerating or decelerating operation
Further, on the other hand, the larger an outer diameter of a roller included in the pair of rollers is made, the larger a load for the driving source becomes, due to increase of the mass of the roller, resulting in an increase of the step out frequency of the stepping motor.
Accordingly, since the sufficient stiffness and the accuracy is necessary for the pair of rollers to be configured into the conveyance mechanism as mentioned in the above, conventionally, the roller made of stainless metal, such as a stainless steel roller on which the paper sheet conveyance surface is directly formed as a stainless surface or a rubber-coated stainless steel roller in which a rubber layer is formed on a shaft member made of the stainless steel, is employed as such the roller.
However, in order to achieve a further minimization and a cost reduction of the apparatus, and to achieve an improvement of the efficiency of the image forming apparatus, etc., by increasing the conveyance velocity of paper sheet, it has been quite difficult to fulfill the abovementioned conditions when employing the conventional pair of rollers made of stainless metal. Accordingly, the prevention of step out has been the subject to be solved.